Portable wireless devices are typically operated on battery power, which provides a finite operating life before the battery must be recharged or replaced. The drain on the battery varies depending upon the operating mode of the device. Relatively high power levels may be required used when the device is transmitting signals to other devices or receiving and processing signals from other devices. In existing systems, devices may be shifted between an awake and a sleep mode in order to conserve power. These awake and sleep modes typically correspond to fixed periods, such as every few superframes or beacon periods. As a result, the devices are in the awake mode for entire beacon periods even though the device may need to receive or transmit frames for only a portion of the beacon period. The remaining time between such receive or transmit processing is essentially a waste of the device's power.
One reason for keeping a device in the awake mode for an entire beacon period is that the device may not know when unscheduled frames may be received from a hub or cluster controller. The device cannot afford to transition to a sleep mode because it may miss such unscheduled frames. Accordingly, there is a need for a medium access selection process that allows for more accurate control of the awake and sleep modes of a wireless device to minimize power consumption.